Balance of Thymic Negative Selection vs. Treg Cell Generation in the Elderly Abstract Age-related thymic involution builds up a predisposition to autoimmunity that contributes to chronic inflammation in the elderly (termed inflamm-aging). This is due to a defect in negative selection which results in increased release of self-reactive T cells. Inflamm-aging significantly increases mortality and morbidity from cardiovascular and neurodegenerative diseases, and late-life cancer in the elderly. FoxP3+ regulatory CD4 T cells (Tregs) act as suppressors to inhibit self-reactive T cell-induced autoimmunity and chronic inflammation. Although aged individuals usually exhibit an accumulation, instead of reduction, of Tregs in the periphery, it is largely unknown whether Treg generation in the atrophied thymus is reduced or enhanced, how it is balanced with perturbed negative selection, and how it suppresses increased self-reactive T cells in the aged periphery. We hypothesize that the atrophied thymus attempts to balance defective negative selection by enhancing Treg generation to establish immune tolerance in aged individuals, and accumulation of Tregs in the aged periphery is probably another way to balance increased self-reactive T cells. There is a critical need to establish this knowledge base in this field in order to efficiently control inflam-aging and age-related autoimmune predisposition, thereby reducing the risk of age-related diseases. To this end, we design three aims to address these knowledge gaps. Aim 1: Establish evidence for the mechanism by which immunotolerance is balanced via enhanced Treg generation to compensate for defects in negative selection in the aged thymus; Aim 2: Determine why enhanced Treg generation in the aged thymus cannot sufficiently suppress self-reactive T cell-induced inflamm-aging in the aged periphery; Aim 3: Explore a potential rejuvenation strategy using induced thymic epithelial-like cells (iTECs) to re-balance negative selection vs. Treg generation in the atrophied thymus and attenuate inflamm-aging in the aged periphery. Upon completion of these studies, a novel mechanism involving a balance of self-reactive T cells vs. specific Treg cells, which when disrupted leads to age-related autoimmune predisposition and inflamm-aging, will be established. These mechanistic insights will serve as a base for the development of clinically significant rejuvenation strategies to restore abnormalities in the T cell immune system in the elderly and attenuate autoimmune predisposition-associated inflamm-aging.